The present invention relates to roentgenography in general, and more particularly to tomography, i.e., to a procedure of making a detailed X-ray image of details in a selected plane of an object located between the source of X-rays and the charge-receiving material which is to be exposed to object-modulated X-rays. Still more particularly, the invention relates to the making of tomographic images of X-rayed objects by resorting to sheet-like receptors which are confined in the gas-filled interelectrode gap of an ionography imaging chamber during exposure to a pattern of X-rays.
A normal (i.e., conventional) X-ray image furnishes the sum of absorption differences of all layers or strata of an object which is located between the source of X-rays and the charge-receiving sheet. On the other hand, a tomographic image furnishes sharp details of a single stratum of the X-rayed object. The thickness of the selected stratum can be varied within a reasonably wide range. The details in the other stratum or strata of the object are blurred.
In making a conventional tomographic image, the object is stationary while the source of X-rays and the X-ray film are moved relative to the object and relative to each other (in opposite directions) in such a way that portions of a selected layer of the object are imaged on the same portion of the film. The images of portions of other layers of the object constantly change their positions relative to the film so that the outlines of images of such portions are unsharp. Presently known tomographic apparatus are operated to continuously change the angle between the central beam of the bundle of X-rays and the film plane. This does not appreciably affect the quality of images which are exposed onto a silver-containing photosensitive layer. However, if the film is replaced with a sheet-like receptor of object-modulated X-rays which is confined in the interelectrode gap of an ionography imaging chamber, the sharpness of the tomographic image is highly unsatisfactory. This is due to the fact that the width of the interelectrode gap is normally in the range of one centimeter. Thus, when the inclination of the gap with respect to the selected layer of the object changes (simultaneously with changes in the direction of penetration of X-rays), the X-rays generate ion trails which are not parallel to the electric field that develops between the electrodes of the ionography imaging chamber. Consequently, each spot or unit area of the selected layer of the object is imaged in the form of a line.